Inductive charging uses an electromagnetic field to transfer energy between two objects through electromagnetic induction. Energy is sent through an inductive coupling to an electrical device, which can then use that energy to charge batteries or run the device. Induction chargers use a primary induction coil to create an alternating electromagnetic field from within a charging base, and a secondary induction coil in a portable device takes power from the electromagnetic field and converts it back into electric current to charge the battery. However, this technology requires that the two coils be positioned close to each other to reduce transfer losses. As such, it restricts the distance between the charging base and the portable device. For example, in the application of wirelessly charging a smartphone, the smartphone must be left on a charging base, and thus cannot be moved around or easily operated while charging.
Resonant inductive coupling is the near field wireless transmission of electrical energy between two magnetically coupled coils that are part of resonant circuits tuned to resonate at the same frequency. A resonant circuit, also called LC circuit or tank circuit, is an electric circuit including an inductor (also known as coil) and a capacitor. FIG. 1 illustrates a typical example of such a resonance-based wireless power transfer system. As shown, the power transmitter includes a power source 101, a resister 102, a capacitor 103, and a transmitter coil 104. Here, the capacitor 103 and the transmitter coil 104 form a parallel circuit (also known as parallel LC circuit). The power receiver includes a receiver coil 105, a capacitor 106, and a resistor 107, and the capacitor 106 and the receiver coil 105 are connected in parallel as well. During operation, the two coils 104 and 105 form an inductive link, through which electrical energy is wirelessly transmitted from the transmitter to the receiver. FIG. 2 illustrates another example of a resonance-based wireless power transfer system, where the power transmitter's capacitor 202 and coil 204 are connected serially (also known as serial LC circuit) and same are the power receiver's coil 205 and capacitor 206.
Prior resonance-based wireless power transfer systems were limited to a fixed distance and orientation, with efficiency falling off rapidly when the distance and/or orientation between the transmitter coil and receiver coil (such as the ones shown in FIG. 1 or FIG. 2) change from their optimal operating point. Some resonance-based wireless power transfer systems use adaptive frequency tuning method to overcome the above problems. For example, US Patent Publication No. 20090284220 discloses a wireless power transfer system whose transmitter and receiver are tuned to resonate at the same frequency. In addition, the transmitter and the receiver each contain additional control circuit to tune its own frequency when it detects a “mismatch.” This solution, however, introduces additional circuits, which not only consume extra power but also make the transmitter and receiver bulky.
Resonance-based wireless power transfer systems have also been designed to have the power transmitter and receiver resonating at the same frequency so that greater distance between the transmitter and receiver may be achieved. For example, “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” Andre Kurs, et al., SCIENCE, VOL 317, pp 83-85, 6 Jul. 2007 discloses that it is essential that the transmitter and receiver be resonating at the same frequency, and otherwise the efficiency of power transmission will drop sharply. Similarly, a research paper by UCLA (available at http://escholarship.org/uc/item/5fz2p58z#page-1) discloses a wireless power transfer mechanism which can achieve stable power delivery over distance variation and high power transfer efficiency. The paper suggests that to achieve maximum power transfer the transmitter and the receiver must have the same resonant frequency. Thus, prior resonance-based wireless power transfer systems emphasize on the condition that the transmitter circuit and the receiver circuit resonate at the same frequency.
However, it is difficult to accurately control an LC circuit's resonant frequency in mass production because an LC circuit's resonant frequency depends on the inductance of the coil in the circuit and the inductance of a coil depends on various factors, such as the number of turns, separation of the turns, the geometrical size of each turn, the geometrical shape of the coil, and the magnetic permeability of nearby materials. Thus, to make sure each power transmitter or receiver resonates at a particular frequency, extensive testing and fine tuning are required during manufacturing. This complicated testing and tuning process increases production cost and decreases production yield. Furthermore, because a coil's inductance may be affected by its operating environment (e.g., the magnetic permeability of nearby materials) a power transmitter or receiver may resonate at a frequency different from its original resonant frequency when manufactured.
In addition, even if a power transmitter and a power receiver have the same resonant frequency f0, they may reach to one of three different stable oscillating states when coupled together: (1) oscillating at their original resonant frequency f0, (2) oscillating at a frequency f1, that is slightly lower than f0, or (3) oscillating at a frequency fH that is slightly higher than f0. It is difficult to predict and control which one of the three oscillating states the transmitter and receiver will reach to when they become coupled or when the distance between them changes. And at frequency f0, both the energy transfer efficiency and range degrade significantly.
Thus, a new wireless power transfer system that can overcome the above shortcomings of prior wireless power transfer systems is desired.